Many disk drives include a spare digital-to-analog converter (DAC) that can be used to scale and display a position error signal. The position error signal can be used to determine a current needed to center a disk read head over a target disk track. Because disk tracks are not perfectly circular, the position error signal is sampled many times per revolution to keep the disk read head centered.
FIG. 1A is a schematic block diagram of a disk drive 10 that uses a conventional A-B position signal. The disk drive 10 includes a servo demodulator 20, analog servo system compensator filter 30, and actuator driver amplifier 40. The servo demodulator 20 receives an A-B position signal from a disk digital-to-analog converter. The position signal may be in units of volts/track distance. The servo demodulator 20 performs an analog peak detect for servo bursts commonly referred to as A, B, C, and D.
FIG. 1B illustrates a method of determining a position error signal for an analog servo disk drive with reference to FIG. 1A will now be described. The first step, step 50, involves reading an A-B signal from a disk. The A-B signal is typically in digital form. Step 60 involves demodulating the amplitude of the A-B signal from the disk using a servo demodulator. The A-B signal is converted from a digital signal into an analog signal in step 70. Next, step 80 involves an analog amplifier filter for a servo system compensator that filters the A-B signal. In step 90, a voltage to current amplifier driving a position actuator amplifies the A-B signal.
In a disk drive with an A-B type position signal, a maximum position error signal that can be represented is ½ track. The position error signal is then inverted in step 100 to represent the signal as an all-positive representation. The inversion changes a negative portion of the signal to a positive portion essentially causing a polarity sign reversal. The sign reversal causes the system to be unstable and limits the acceptable range position servo.
Some processors neither have access to certain signals nor enough processor speed to calculate a composite position signal (made up of grey code, A-B and C-D signals) to create an extended position signal. In addition, if the extended position error is output with early saturation into an analog compensator amplifier, a very nonlinear signal results that tends to be unstable.
Accordingly, there exists a need to extend the position error signal in a typical disk drive storage system to be compatible with a limited microprocessor and an analog servo system.